The CV value of a valve is a key value to measure its flow capacity! It tells you how much flow the valve can pass under a specific pressure difference. Mastering a simple calculation method is the first step to accurate model selection and avoiding system problems. This article teaches you how to calculate the CV value in the most understandable way.
What is the valve CV value?
The valve CV value is the core parameter for measuring the flow capacity of the valve – it is like a ruler that intuitively tells you “how much flow can flow when the valve is opened to a certain extent”.
To put it more simply: imagine a faucet. The CV value is similar to a standard value that describes the relationship between the opening of this “faucet” and the size of the water flow. The larger the CV value, the greater the flow rate of fluid (liquid or gas) that can pass through the valve under the same pressure difference conditions, that is, the wider the “passage” of the valve, the stronger the flow capacity. It is the key basis for engineers and purchasers to determine whether the valve can meet the system flow requirements when selecting the valve.
CV value calculation formula (core)
The most commonly used and core valve CV value calculation formula is:
CV = Q * √(SG / ΔP) (mainly used for liquid calculations)
The meaning of this formula is: The flow capacity (CV value) of the valve under specific conditions is equal to the flow rate (Q) flowing through the valve multiplied by the specific gravity of the fluid (SG) divided by the square root of the pressure difference before and after the valve (ΔP).
Simple understanding:
- The larger the flow rate (Q), the larger the CV value required (the wider the valve passage).
- The greater the pressure difference (ΔP), the smaller the CV value required (the greater the pressure difference, the easier it is to push the fluid through a small channel).
- The greater the specific gravity (SG) of the fluid (heavier than water), the larger the CV value required (a larger channel is required to push heavy fluids).
Formula symbols and unit description
| Symbol | Representative meaning | Common units | 설명 |
| CV | Flow coefficient | Dimensionless | The final result to be calculated represents the flow capacity of the valve. The larger the value, the stronger the flow capacity. |
| Q | Fluid flow | US Gallons per Minute (GPM) Cubic meter/hour (m³/h) | Volume flow through the valve. The units must be consistent when calculating! |
| ΔP | Valve pressure difference | Pound force per square inch (psi) bar | Valve inlet pressure minus outlet pressure. Not the total system pressure difference! The units must be consistent! |
| SG | Fluid specific gravity | Dimensionless | The ratio of the density of a fluid to the density of standard water at 4°C. When water is at 4°C, SG=1. |
Key Tips
- It is very important to use the same units! The formula format is the same when using US units (GPM, psi) or metric units (m³/h, bar), but you must make sure that the units of Q and ΔP belong to the same system. Values from different systems cannot be directly substituted!
- Differential Pressure Position: ΔP refers specifically to the differential pressure across the valve itself, not the total differential pressure across the entire piping system or pump.
- Mainly applicable to liquids: This formula is the most basic and commonly used form of calculating the CV value of liquids. The calculation of gases is more complicated (involving temperature and compressibility).
CV value calculation steps (Three steps)
Step 1: Determine the parameters
- Q (flow): e.g. 50 m³/h
- ΔP (valve internal pressure difference): e.g. inlet 5.8 bar – outlet 5.0 bar = 0.8 bar
- SG (Specific Gravity): Water = 1.0 , other fluids refer to the table
Step 2: Unify the unit system (❗Do not mix)
Choose any combination of units:
| parameter | Metric units | or US units |
| Q | m³/h | GPM |
| ΔP | bar | psi |
| SG | Dimensionless (unchanged) | Dimensionless (unchanged) |
Step 3: Substitute the formula into the calculation
Formula: CV = Q × √(SG / ΔP)
Example (Metric):
Q=50 m³/h, ΔP=0.8 bar, SG=1.0
CV = 50 × √(1.0 / 0.8) = 50 × √1.25 ≈ 55.9
Formula symbols and unit description (quick lookup table)
| Symbol | Meaning and units | 핵심 요약 |
| CV | Valve flow coefficient ・Unit: dimensionless | The result value directly reflects the flow capacity ・The larger the CV value, the stronger the valve flow capacity |
| Q | Fluid flow ・Metric: m³/h (cubic meters per hour) ・US: GPM (Gallons per minute) | Must match the ΔP unit system! ・Example: Q is expressed in m³/h, then ΔP must be expressed in bar |
| ΔP | Pressure difference before and after valve ・Metric system: bar ・US: psi (pounds per square inch) | Only refers to the pressure difference of the valve itself! ・Calculation formula: Inlet pressure – outlet pressure |
| SG | Fluid specific gravity ・Unit: dimensionless | Water = 1.0 (base value) ・Gasoline ≈ 0.72, concentrated sulfuric acid ≈ 1.84 |
Instructions for use (❗Must read)
- Unit lock: select metric (m³/h + bar) or US (GPM + psi), no mixing
- ΔP Trap: Not the total pressure difference of the system, but only the pressure difference between the two ends of the valve
- SG Reference: When the specific gravity of the fluid is unknown, consult the Material Safety Data Sheet (MSDS)
Notes on applying CV values (must read)
| Project | Key Points |
| Selection margin | Calculate CV value × 1.2~1.5 times for model selection (anti-blocking/flow fluctuation) |
| ΔP measurement point | The pressure difference is taken from the straight pipe section at the inlet and outlet of the valve (avoid elbow/pump interference) |
| Gas calculation | Gases require special formulas (including temperature/compression coefficient), liquid formulas are not applicable! |
| Viscosity correction | The CV value of high viscosity fluids (such as oils) needs to be multiplied by the correction factor (check the valve manual) |
자주 묻는 질문
Q1: Can the CV value of gas be calculated using the liquid formula?
→ No! Gases require complex formulas (including temperature/compressibility), so it is recommended to consult the valve manufacturer directly.
Q2: How to calculate the CV value when multiple valves are connected in series?
→ The total CV value of the system is calculated as the sum of the reciprocal squares:
1/CV²ₜ = 1/CV₁² + 1/CV₂² + … (Each valve must be calculated separately when selecting)
Q3: What is the difference between CV value and Kv value?
→ Same in nature, only the units are different:
CV (American Standard): GPM value of water flow under 1 psi pressure difference
KV (European standard): water flow in m³/h at 1 bar pressure difference
→ Conversion: CV ≈ 1.16 KV
Q4: What should I do if there is no accurate pressure difference data?
→ Method 1: Estimate valve ΔP by pump head × 30% (emergency solution)
→ Method 2: Must be measured (key parameters for selection, cannot be estimated!)
Q5: How to select a valve after calculating the CV value?
→ Check the valve flow curve: select the model with rated CV ≥ calculated value × safety factor (1.2~1.5)
Learn more about CV flow knowledge 《Detailed Explanation of CV Flow Coefficient: Calculation Formula, Industrial Application and Core Value》
As a 60-year API/EN/ISO fully certified valve manufacturer, JH Valve strictly implements international standard production processes and implements triple strict control on the CV value of each valve: ① Accurate calculation according to working conditions during the design stage → ② CV tolerance ±3% (exceeding ISA standard) during manufacturing → ③ Factory water test to verify zero leakage. We fulfill the “zero leakage promise” with vertical customization capabilities (cryogenic/ultra-high pressure/sterile grade), and 24/7 global technical support to ensure that your system operates safely and efficiently throughout its life. Choosing JH means choosing reliability!
